This invention relates to a method for displaying a gray scale for a display and a display device whose brightness can be controlled by varying the amplitude of the voltage applied thereto, and in particular to a gray scale displaying device suitable for a TFT (Thin Film Transistors) liquid crystal display.
Prior art devices for the liquid crystal active matrix display, which consists of 3-terminal switching elements represented by TFTs or 2-terminal non-linear switching elements represented by MIMs (Metal Insulator Metal) and a liquid crystal layer superposed thereon for displaying images are described in JP-A-61--128292 (FIG. 1), JP-A-61--116334 (FIG. 2) and SID 84 DIGEST (1984) pp. 304 307 (FIG. 3).
The signal side circuit of the liquid crystal display indicated in FIG. 1 is composed of a shift register 23, a latch circuit 24 and a group of 2-value switching elements 25.
The group of 2-value switching elements 25 is constituted by one-out-of-two multiplexers, each of which selects either one of the V.sub.D or GND levels and outputs it as a signal voltage.
This signal voltage is inputted in liquid crystal elements 26b through TFTs 26a in a TFT liquid crystal panel 26. The brightness of a liquid crystal element 26b varies according to the level of the signal voltage. As the result, an image is displayed on the TFT liquid crystal panel 26.
However, by this driving method, in order to obtain a number of gradations in brightness greater than 2, the signal voltage should have a plurality of levels corresponding to the number of gradations. Consequently, it is necessary to switch the voltage levels by using a great number of switches.
For this reason this method is disadvantageous, not only because the construction of the signal circuit is complicated but also because the cost for ICs increases, when the signal circuit is constructed by using ICs.
On the other hand, FIG. 2 indicates a prior art example of the intermediate gradation displaying driving circuit A video signal V.sub.s (analogue voltage) is time-sequentially sampled by a sampling holder 29 and inputted in TFTs 32a through a buffer circuit.
For example, for a TV display, when it is assumed that the number of pixels in the horizontal direction of a TFT liquid crystal panel 32 is 500, the sampling time of the sampling switch 29 is about 0.1 .mu.s. Consequently, the driving frequency of a shift register 28 is 10 MHz. When the video signal is sampled with such a high speed, the sampling is imperfect and it is not possible to obtain any uniform display.
Further, for a highly fine display having a number of displaying pixels of 1000.times.1000 the driving frequency of the shift register 28 is further increased, which gives rise to a problem such as cost increases, etc., specifically when it is composed of ICs.
FIG. 3 shows another prior art example, in which reference numeral 33 is an X-driving circuit; 34 is a Y-driving circuit; 35 is an MIM two-terminal element; 36 is a liquid crystal element; and 37 is an electric switch.
A display signal inputted in the X-driving circuit 33 is sampled successively by the electronic switch 37 during one horizontal scanning period and applied to the MIM two-terminal element 35.
If the display signal were a video (analogue) signal, a gray scale display would be possible. However, by this method, when the number of pixels in the horizontal direction is increased, the sampling time is shortened. As the result, an insufficiently high voltage is applied to the liquid crystal element 36, which gives rise to problems such as lower contrast, lack of a uniform display, etc.
In other words, in the prior art gray scale displaying methods, the sampling frequency for the video signal is too high, and further the construction of the output stage of the driving circuit is complicated, which gives rise to problems, specifically when a circuit for driving a highly fine display should be integrated.